Water bath development was used to tame highlights, not strengthen shadows. In my experience if you're underexposed, there's nothing you can do to get back that shadow detail. The whole concept of "push development" is (to me) a myth. Whenever I've tried it I get blown highlights with no effect on the shadows.

In any case, water bath development won't help you, for two reasons: a) when it worked the effect it had was to keep the highlights from blowing out without losing the shadows and b) it only worked with thick emulsion films. So unless you shoot Super XX Pan, the physics of the situation precludes your gaining any advantage from water bath development.

The situation in which it does work today is in printing on Azo, where it is a very effective technique indeed. But of course it works in reverse from what you'd do with film: you place it in the water bath to hold back the shadow densities while allowing the highlights to fully develop.

The only other possibility is stand or semi stand development, which, with the right developer (Rodinal works well) will optimize the low end detail while holding back the highlights. And c6h6o3 makes a good distinction about the purpose of water bathing, and I agree with his take on "push" processing. I have tried it over the years with different films and dev's, also saw results done by students back when I was teaching. Shadows never improved. Sometimes, however, mid to 3/4 tones may move up the curve and may benefit from better separation. But highlights can block up.
Search this forum for "stand" or "semi-stand" and you'll find plenty to read. Pay attention to Don Cardwell's posts.

I second the recommendation for stand/semi-stand development. This will pull as much out of the shadow density as there is there. You can generally get from 1/2 to one full stop in effective film speed with this type of development.

The only other possibility is stand or semi stand development, which, with the right developer (Rodinal works well) will optimize the low end detail while holding back the highlights. And c6h6o3 makes a good distinction about the purpose of water bathing, and I agree with his take on "push" processing. I have tried it over the years with different films and dev's, also saw results done by students back when I was teaching. Shadows never improved. Sometimes, however, mid to 3/4 tones may move up the curve and may benefit from better separation. But highlights can block up.
Search this forum for "stand" or "semi-stand" and you'll find plenty to read. Pay attention to Don Cardwell's posts.

Hi, I just accidentally under exposed one sheet of HP5+ by
one step. So lower shadow values will be troublesome.

That made me wondering that could water bath developing
give any help? With old thick emulsion films it worked, but
how it is with a modern films?

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Local depletion of the developing agent is the mechanism
by which compensation is achieved when using water bath
development. Water bath development though is not the only
way to compensation. As mentioned, stand and semi-stand
development also work.

The developer depletes soon in areas of much exposure while
the less exposed areas continue to increase in density. Very
active highly dilute developers of the correct developing
agent are compensating developers. Metol specifically
is effective due to it's retardation by the local build
up of bromide in the most exposed areas.

Divided developers offer compensation; Stockler's and divided
D-23 come to mind. I've been using D-23 at a 1:7 dilution;
500ml, one 120 roll. With 2 or 3 inversions every 2 or 3
minutes I've managed to pull full speed and maybe a
little more from the Acros film developed. Dan

There was a theory and only a theory that on any exposure all the image was recorded on the film.But it was at such a low level it was below the threshold that would develop, something like that So a technique in vogue in the 40s ,called I believe latent intensification , was tried .It was basically post fogging the film. Prefogging the film had already been used to boost the films speed  for low light levels. and reduce contrast. The movie studios had highly controlled techniques for such conditions and amateurs tried variations. Some claiming enormous speed increases. These of course were all with the old thick emulsions and as such might have had some validity But then we also soaked the film overnight in mercury vapors (hyper sensitization) too to boost film speed and lived to tell about it.

In addition to semi-stand / reduced agitation methods, you could go with a developer that gives higher effective film speeds. Among those would be Xtol, Ilford DDX, Microphen, and Diafine. I'm certain others will be suggested.

Some commonly used semi-stand or stand developers such as Rodinal don't yield as much film speed. You can use the developers that give higher film speed as semi-stand developers.

I can't believe I didn't think of SS development. In my particular case it will actually reclaim a full stop of shadow density, because I rate my film at half box speed for normal tray development. But with SS I get full box speed. So if you're in the habit of rating your HP5+ at 200, SS development will indeed reclaim the full stop of shadow density.